This thesis begins with the origin and development of cadenza. Then the structure of W.A.Mozart¡¦s cadenza is discussed. Mozart¡¦s Piano Concerto K.V.453 first movement is used as an example to explain the ritornello form and the cadenza structure. Finally, those of other composers, such as Johannes Brahms (1833-97), Ferruccio Busoni (1866-1924), Ernst von Dohnányi (1877-1961) and Edwin Fischer (1886-1960), their cadenzas for K.V.453 first movement will also be analyzed and compared. Most of Mozart¡¦s cadenzas can be analyses as three-part form: The first part starts either themes from the concerto movement or virtuoso runs which appeared in the movement before or newly invented. The second part usually over a sustained bass note or chord continually develops sequences of an important theme or motive from the concerto movement. This will leads into the third part with a number of virtuoso runs, passages in thirds, etc., until the close of the cadenza which usually ends with a long trill. During the 19th and 20th centuries, performers performed Mozart Piano Concerto, seldom adopted the cadenza of Mozart's. Rather, they would replace Mozart's cadenza with those written by themselves. Those cadenzas according to the treatises and Mozart¡¦s own cadenzas usually did not have the so-called ¡§Mozart's style¡¨. Because the authenticity movement of performing early music in the late 20th, when a performer decides to perform Mozart Piano Concerto, it becomes very important for him to figure out how to choose or create the proper cadenza for himself to perform. Yet, the change of time with different personality and style will influence the result of creating and performing a cadenza. Maybe this is one the main reason that make the cadenza so very and interesting always.